A Mathematical Model for the Release, Transport, and Retention of Per‐ and Polyfluoroalkyl Substances (PFAS) in the Vadose Zone

Abstract

Per‐ and polyfluoroalkyl substances (PFAS) are emerging contaminants of critical concern. As surfactants, PFAS tend to accumulate at air‐water interfaces and may stay in the vadose zone for long times before contaminating groundwater. Yet not well understood, the extent of retention in the vadose zone has critical implications for risk management and remediation strategies. We present the first mathematical model that accounts for surfactant‐induced flow and solid‐phase and air‐water interfacial adsorption. We apply the model to simulate PFOS (a PFAS compound of primary concern) transport in the vadose zone at a model fire‐training area site impacted by aqueous film‐forming foam (AFFF). Air‐water interfacial adsorption is shown to have a significant impact—amplified by the low water content due to gravity drainage—total retardation factors range from 233 to 1,355 for the sand and 146 to 792 for the soil used in the study. The simulations illustrate it can take several decades or longer for PFOS to reach groundwater. Counterintuitively, the lower water content in the sand—due to stronger drainage and weaker capillary retention—leads to retardation factors greater than for the soil. Also, most PFOS is adsorbed at air‐water interfaces with only 1–2% in the aqueous phase. The implications include (1) fine‐texture materials could have lower retardation factors than sand due to higher retained water content, (2) soil PFAS concentrations are likely to be orders of magnitude higher than those in groundwater at source zones. Both implications are consistent with recent field observations at hundreds of AFFF‐impacted sites.

Multidimensional simulation of PFAS transport and leaching in the vadose zone: impact of surfactant-induced flow and soil heterogeneities

“…The numerical simulations of PFAS leaching in the vadose zone reported by Guo et al (2020) are generally consistent with soil PFAS concentration profiles measured at contaminated sites.…”

“…Numerical simulations of PFOS (Perfluorooctanesulfonic acid, C8HF17O3S) released at a model aqueous film-forming foam (AFFF)-impacted fire training area (FTA) site and the subsequent transport and retention in the vadose zone demonstrated that PFOS can be strongly retained in the vadose zone even under highly transient variably saturated flow driven by timedependent rainfall infiltration. Shortly after, a model similar to that of Guo et al (2020) was also independently reported by Silva et al (2020aSilva et al ( & 2020b, where example simulations were presented in both 1D and 2D domains.…”

“…Many PFAS are surfactants (Kissa, 2001), thus they tend to adsorb at air-water interfaces and solid surfaces in soils and may stay in the vadose zone for long times before contaminating groundwater (Brusseau, 2018;Guo et al, 2020). Using surface tension data of two primary PFAS and measured air-water interfacial (AWI) areas in soil materials, Brusseau (2018) examined the impact of PFAS adsorption at air-water interfaces as a potential retention process in soils.…”

“…Mathematical modeling of PFAS transport and retention under field conditions in the vadose zone has only begun recently. Guo et al, (2020) reported a new mathematical model that represents a comprehensive set of PFAS-specific transport processes including surfactant-induced flow (SIF), nonlinear and rate-limited solid-phase and AWI adsorption under transient variably saturated water flow. Numerical simulations of PFOS (Perfluorooctanesulfonic acid, C8HF17O3S) released at a model aqueous film-forming foam (AFFF)-impacted fire training area (FTA) site and the subsequent transport and retention in the vadose zone demonstrated that PFOS can be strongly retained in the vadose zone even under highly transient variably saturated flow driven by timedependent rainfall infiltration.…”

Application of PFAS‐mobile lab to support adaptive characterization and flux‐based conceptual site models at AFFF releases

“…It is noted that this generic DAF was developed as a conservative approach to estimating groundwater concentrations associated with leaching from vadose zone soils based on a simple mass balance approach that compares source mass loading to groundwater flow and mixing with depth. Clearly, the SPLP is conservative in that it represents a saturated batch extraction whereby all air–water interfacial retention (see Brusseau et al, 2019; Guo et al, 2020) is eliminated with accelerated desorption as a result of aggressive agitation (e.g., Kalbe et al, 2008). Thus, SPLP results should not necessarily be expected to simulate actual porewater concentrations.…”